The industry that is associated with the drilling of oil and gas wells has long used hydraulically-actuated tools such as packer or anchor assemblies to support other tools in the borehole. One such tool used in conjunction with anchors or packers is a whipstock. A whipstock includes an inclined face and is typically used to direct a drill bit or cutter in a direction that deviates from the existing wellbore. The combination whipstock and anchor (or packer) is frequently termed a sidetrack system. Sidetrack systems have traditionally been used to mill a window in the well casing, and thereafter to drill through the casing window and form the new borehole.
Originally, such a sidetrack operation required two trips of the drill string. The first trip was used to run and set the anchor or packing device at the appropriate elevation in the borehole. With the anchor or packer in place, the drill string was then removed from the well and a survey was made to determine the orientation of a key on the upper end of the anchor-packer. With that orientation known, the whipstock was then configured on the surface so that when the whipstock engaged the anchor-packer in the borehole, it would be properly oriented. So configured, the whipstock, along with an attached cutter, was then lowered in the wellbore on the drill string and secured to the anchor-packer. Once connected to and supported by the packer, the whipstock directed the cutter so that a window would be milled in the casing of the wellbore at the desired elevation and in the preselected orientation. As is apparent, this two-trip operation for setting the anchor-packer and then lowering the whipstock and cutter is time-consuming and expensive, particularly in very deep well operations.
To eliminate the expense associated with two trips of the drill string, an improved sidetrack system was developed which required only a single trip. Such a system is described, for example, in U.S. Pat. Nos. 4,397,355 and 4,765,404 and includes a whipstock having an anchor-packer connected at its lower end, and a cutter assembly releasably connected at its upper end. Using such a system, the whipstock is oriented by first lowering the apparatus into the cased wellbore on a drill string. A wireline survey instrument is then run through the drill string to check for the proper orientation of the suspended whipstock. In generally vertical wellbores, the wireline tool typically can be lowered in the drilling mud by gravity alone. In heavier muds, however, or in wellbores which deviate from vertical to a significant degree, it is frequently necessary to circulate the drilling mud through the drill string in order the pump the wireline tool from the surface to the whipstock.
To permit the circulation required to transport the wireline sensing device down to the whipstock, prior art systems have included a bypass valve which would allow drilling mud at relatively low flow rates (typically less than 100 g.p.m) to circulate through the drill string without setting the hydraulically-actuated anchor-packer. Once the wireline sensor has been transported by the circulating drilling mud to the location required for detecting the orientation of the whipstock, and after the whipstock is properly oriented in the borehole, the bypass valve could then be closed and the drill string pressurized so as to actuate the anchor-packer. With the anchor-packer set, the drill string is then lowered causing the cutter assembly to become disconnected from the whipstock. As the cutter is lowered further, the inclined surface of the whipstock cams the rotating cutter against the well casing, causing the cutter to mill a window in the casing at the predetermined orientation and elevation.
While the single-trip method and apparatus described above is an improvement over the prior two-step system, it nevertheless suffers from significant drawbacks. As mentioned above, it is many times difficult to transport the wireline sensor into the position that is required for detecting the orientation of the drill string when drilling with heavy drilling muds. It has likewise been found to be quite difficult to transport the wireline sensor and have it properly engage the whipstock assembly in wellbores that deviate significantly from vertical. Because in today's drilling industry, where steerable systems are frequently employed to drill holes horizontally or at angles that even exceed horizontal, it should be appreciated that this inability to properly land or connect a wireline device is a very significant drawback to using the technology that is presently available.
To be contrasted with wireline devices, there exist today a variety of systems that are capable of collecting and transmitting data from a position near the drill bit while drilling is in progress. Such measuring-while-drilling ("MWD") systems are typically housed in a drill collar at the lower end of the drill string. In addition to being used to detect formation data, such as resistivity, porosity, and gamma radiation, all of which are useful to the driller in determining the type of formation that surrounds the borehole, MWD tools are also useful in surveying applications, such as, for example, in determining the direction and inclination of the drill bit. Present MWD systems typically employs sensors or transducers which, while drilling is in progress, continuously or intermittently gather the desired drilling parameters and formation data and transmit the information to surface detectors by some form of telemetry, most typically a mud pulse system. The mud pulse system creates acoustic signals in the drilling mud that is circulated through the drill string during drilling operations. The information acquired by the MWD sensors is transmitted by suitably timing the formation of pressure pulses in the mud stream. The pressure pulses are received at the surface by pressure transducers which convert the acoustic signals to electrical pulses which are then decoded by a computer.
MWD tools presently exist that can detect the orientation of the drill string without the difficulties and drawbacks described above that are inherent with the use of wireline sensors. It would thus at first seem advantageous to use such MWD tools in a sidetrack system to orient a whipstock and set a packer, or to actuate any other type of hydraulically-actuated downhole mechanism where achieving a particular orientation is important. Unfortunately, known MWD tools typically require drilling fluid flow rates of approximately 250 gallons per minute to start the tool, and 350 to 400 gallons per minute to gather the necessary data and transmit it to the surface via the mud pulse telemetry system. The conventional bypass valves used in present-day sidetrack systems for circulating drilling fluid and transporting a wireline sensor to the whipstock tend to close, and thereby actuate the anchor-packer, at flow rates of approximately 100 gallons per minute, or even less. Thus, while it might be desirable to combine MWD sensors in a sidetrack system, if drilling mud was circulated through the drill string at the rate necessary for the MWD tool to detect and communicate to the driller the orientation of the whipstock, the bypass valve would close and the anchor-packer would be set prematurely, before the whipstock was properly oriented. Thus, despite the theoretical advantage which an MWD tool could provide in orienting and setting a hydraulically-actuated mechanism, a system presently does not exist to take advantage of the benefits that an MWD tool might provide.